am i mad for even thinking about this?

I got a 60W bass combo, with an additional 40W speaker output, the thing that just popped in my head whas hooking up 2 HiFi speakers that aren't used anyway at the moment on that 40W output, they got 1 x 10" bass driver each with quite a good bassresponse for a 3way HiFi speaker (down to 40Hz flat/ 35Hz +/-5 dB)

The crazy idea behind this is to use those external speakers as some sort of "monitor" for the band (rehersals) while I use the combo.

Consumer home stereo speakers are very inefficient. They are often as low as 85 dB SPL @ 1W1M. Also, they aren't designed for non-program material. This means that they are not designed to take the wide dynamic range of live bass playing. (Recorded material is compressed to reduce the dynamic range.) So in the end you will likely get poor volume and trashed speakers.

B Funk's right. Have a look at the speaker itself and i bet it's got a flimsy foam surround. If you play at a decent volume you're likely to shred the speaker. Plus with sensitivity ratings 10db lower than the other cab, you probably won't even be able to hear it.

My first attampt at a home made cab used 2x10" car subs (glorified Hi-Fi) and it was a dismal failure. Sounded fine on it's own but put it next to a proper bass cab and all the inadequacies became very obvious. You live and learn.